The present invention relates generally to timing synchronization in communication systems and, more particularly, to timing synchronization for communication systems including radio communication links.
It is often advantageous in a communication network for devices in the network to have a common time base. For example, in 3G mobile/cellular communication systems, radio base stations (RBS) are synchronized to within a few microseconds. Precise synchronization facilitates efficient communication such as for allocation of time slots among devices and interference reduction. The electronics industry has developed several standard protocols for use in synchronizing clocks, for example, the Precision Time Protocol (PTP) of IEEE Std. 1588. PTP includes sending timing-related messages between nodes in a communication network. The timing-related messages include, for example, a first node transmitting over a link a time-stamped packet to supply its time base to a second node, and the second node transmitting, over the link, a packet to the first node requesting and thereafter receiving a reply with the time of receipt of the request packet, so that the second node may have information regarding packet time of travel over the link. With these timing related messages the second node may be able to synchronize its local clock to the clock time of the first node, with it often being assumed that packet travel time over the link between the nodes is constant.
Mobile communication networks often use radio links, for example at microwave or millimeter-wave frequencies to provide backhaul from radio base stations to a base station controller or gateway reached through a carrier network. Delivering high-accuracy synchronization using packet-based techniques such as IEEE Std. 1588 (2008), incorporated by reference herein, is especially challenging over radio links. Radio systems have progressed in both capacity and cost. However, many systems may use proprietary communication protocols. The protocols may not include features for establishing common time bases. Moreover, radio systems may have variable delays in propagating information, for example due to changes in bitrate and modulation scheme used for transmission over the wireless radio link. In addition, wireless backhaul equipment may include switches, for example Ethernet switches, and data passing through such switches may encounter variable delays. Thus, timing synchronization of mobile communication systems connected by microwave radio links may be difficult.